I. Field of the Disclosure
The technology of the disclosure relates generally to fabrication of magnetic tunnel junction (MTJ) elements in semiconductor devices, and more particularly to MTJ etching.
II. Background
Semiconductor storage devices are used in integrated circuits (ICs) in electronic devices to provide data storage. One example of a semiconductor storage device is a magnetic random access memory (MRAM). MRAM is non-volatile memory in which data is stored by programming a magnetic tunnel junction (MTJ) as part of an MRAM bit cell. One advantage of an MRAM is that MTJs in MRAM bit cells can retain stored information even when power is turned off. This is because data is stored in the MTJ as a small magnetic element rather than an electric charge or current.
In this regard, an MTJ comprises a free ferromagnetic layer (“free layer”) disposed above a fixed or pinned ferromagnetic layer (“pinned layer”). The free and pinned layers are separated by a tunnel junction or barrier formed by a thin non-magnetic dielectric layer. The magnetic orientation of the free layer can be changed, but the magnetic orientation of the pinned layer remains fixed or “pinned.” Data can be stored in the MTJ according to the magnetic orientation between the free and pinned layers. When the magnetic orientations of the free and pinned layers are anti-parallel (AP) to each other, a first memory state exists (e.g., a logical ‘1’). When the magnetic orientations of the free and pinned layers are parallel (P) to each other, a second memory state exists. The magnetic orientations of the free and pinned layers can be sensed to read data stored in the MTJ by sensing the resistance when current flows through the MTJ. Data can also be written and stored in the MTJ by applying a magnetic field to change the magnetic moment of the free layer to either a P or AP magnetic orientation with respect to the pinned layer.
Recent developments in MTJ devices involve spin torque transfer (STT)-MRAM devices. In STT-MRAM devices, the spin polarization of electrons, rather than the charge of the electrons, is used to indicate the state stored in the MTJ (i.e., a ‘0’ or a ‘1’). FIG. 1 illustrates a STT-MTJ 100. The STT-MTJ 100 is provided as part of an MRAM bit cell 102 to store non-volatile data. A metal-oxide semiconductor (typically n-type MOS, i.e., NMOS) access transistor 104 is provided to control reading and writing to the STT-MTJ 100. A drain (D) of the access transistor 104 is coupled to a bottom electrode 106 of the STT-MTJ 100, which is coupled to a pinned layer 108. A write line (VWL) is coupled to a gate (G) of the access transistor 104. A source (S) of the access transistor 104 is coupled to a voltage source (VS). A bit line (VBL) is coupled to a top electrode 110 of the STT-MTJ 100, which is coupled to a free layer 112. The pinned layer 108 and the free layer 112 are separated by a tunnel barrier 114. When writing data to the STT-MTJ 100, the gate (G) of the access transistor 104 is activated by activating the write line (VWL). A voltage differential between the bit line (VBL) and the source line (VS) is applied. As a result, a write current (I) is generated between the drain (D) and the source (S).
With continuing reference to FIG. 1, if the magnetic orientation of the STT-MTJ 100 in FIG. 1 is to be changed from AP to P, a write current (IAP-P) flowing from the top electrode 110 to the bottom electrode 106 is generated. This induces a spin transfer torque (STT) at the free layer 112 to change the magnetic moment of the free layer 112 to P with respect to the pinned layer 108. If the magnetic orientation is to be changed from P to AP, a current (IP-AP) flowing from the bottom electrode 106 to the top electrode 110 is produced, which induces an STT at the free layer 112 to change the magnetic orientation of the free layer 112 to AP with respect to the pinned layer 108. Note that more write current (I) is required to switch the STT-MTJ 100 from a P to AP state than from an AP to P state in an asymmetrical manner due to the different spin-transfer efficiency at both sides of the tunnel barrier 114 of the STT-MTJ 100. The difference in spin-transfer efficiency relates to the difference in magnetic pole coupling between the pinned and free layers 108, 112 when in the AP and P states. Write asymmetry causes greater operational power consumption by the STT-MTJ 100. In this case, a greater write current would be required to switch the STT-MTJ 100 from a P to AP state than from an AP to P state, thus reducing write current operating margin of the STT-MTJ 100. Thus, to provide for greater write current symmetry in the STT-MTJ 100, the pinned layer 108 can be provided in the STT-MTJ 100 to be physically wider than the free layer 112 such that the magnetic fringe field in the pinned layer 108 is located further away from the free layer 112 to reduce magnetic coupling.
MTJ patterning or etching processes are used to fabricate MTJs. Thus, if it is desired to fabricate the STT-MTJ 100 in FIG. 1 to provide for the free layer 112 smaller in width than the pinned layer 108, a suitable MTJ etching process must be used. Currently known methods for MTJ etching include ion beam etching (IBE) and chemical etching in a reactive ion etching (RIE). RIE processes are known to create damage zones around the perimeter of the MTJ. Etching damage in the transition metals (i.e., the pinned layer 108, the free layer 112, and the bottom and top electrodes 106, 110) in the MTJ can affect factors such as a tunnel magnetoresistance (TMR) ratio and energy bather (Eb) variations, which can result in poor MTJ performance. Also, as MTJs become scaled down, such as in high-density MRAMs, these damages zones limit the amount of downscaling.
Another method of MTJ etching involves ion beam etching (IBE). IBE may be used for etching materials that have tendencies to not react well to chemical etching. An IBE etching process can avoid or reduce damage zones over RIE processes, but no chemical component is involved to improve etching selectivity IBE involves directing a charged particle ion beam at a target material to etch the material. FIG. 2 illustrates an example of an MTJ 116 being fabricated by an IBE process to provide a reduced size free layer for greater write and retention symmetry. As shown in FIG. 2, the MTJ 116 is comprised of an MTJ stack-up 118 of a bottom electrode 120, a pinned layer 122, a tunnel bather 124, and a free layer 126 similar to the STT-MTJ 100 in FIG. 1. To protect areas of the MTJ stack-up 118 that are not to be etched during free layer etching, a hard mask 128 is employed as shown in FIG. 2. The hard mask 128 selectively exposes the desired layers of the material to be etched in the MTJ stack-up 118. The result is a pattern provided in the MTJ stack-up 118 that has been masked from exposure. As shown in FIG. 2, the free layer 126 is etched away to be smaller in width than the pinned layer 122. However, a large footing 132 of the free layer 126 material still remains as a function of the hard mark 128. The footing 132 will contribute to write and retention behaviors. If the strength of an ion beam 130 is increased to try to reduce the size of the footing 132, the tunnel barrier 124 and the pinned layer 122 may also be etched resulting in potential damage and material re-deposition that could shorten the pinned and free layers 122, 126.
To solve these issues, the ion beam could be directed at a large angle of incidence towards an MTJ stack-up, as shown in the alternative MTJ 116′ in FIG. 3. Directing an ion beam at an angle of incidence towards an MTJ stack-up can reduce the size of a footing in a free layer while minimizing etch damage to the layers below the free layer and material re-deposition. In this regard, FIG. 3 illustrates the MTJ 116′ having a smaller footing 132′ in the free layer 126′ of the MTJ stack-up 118′ because of the angle of incidence of the ion beam 130′ directed at the MTJ stack-up 118′. IBE is performed while a semiconductor wafer 134 supporting the MTJ stack-up 118′ is rotated. During the IBE process, adjacent MTJ 116's cast a “shadow” on other MTJs 116′ from certain directions. Thus, the ion beam 130′ in FIG. 3 will be blocked from reaching the MTJ stack-up 118′ from certain directions as the MTJ stack-up 118′ is rotated during fabrication, thus causing an asymmetrical free layer 126′. The shadow-effect will result in greater magnetic coupling between the free layer 126′ and the pinned layer 122 in the MTJ stack-up 118′ in FIG. 3 than would otherwise exist if the ion beam 130′ were not blocked from certain directions during the etching process.